Does If, Else If Require An Ending Else?

For a long time I have been using:

if
else if
else if
else if

For example, I'm using it now to put different ads on different forum pages after checking the forum number in the URL. If there's no match, I don't want an ad on the page.

Is my code above "legal" or does it require an else at the end?

BTW, the reason I do it that way instead of just using ifs is that I figured that once it found a true condition it wouldn't bother reading the following else ifs and that would save time... it would run faster.